Is there a way to exclude an axis from the solidify effect?
You can do it in two steps:

Extrude, but don't move E then RMB
Then push/pull using shiftZ to avoid z displacements

&quot;Push/pull&quot; is available in the tool bar (alternate from shrink/flatten) or via menu &quot;mesh/transform/push pull&quot;.
Though, you'll have to tune the distance with the mouse as the operator panel does not allow to choose the concerned axis.